There are several extant methods and apparatus for the measurement of ultrafiltration in hemodialysis. The principal ones are as follows:
I--Extracorporeal PA0 II--Travenol--negative pressure type PA0 III--Travenol--coil type PA0 IV--Drake Willock--negative pressure dialyzer only
Uses special negative pressure dialyzer (hollow fiber). PA1 Each dialyzer (single use) incorporates chambers and ducts with a 100 ml/hr. to 700 ml/hr. scale. The technique requires: PA1 (1) inverting the dialyzer while the patient is "on"; (2) stopping dialysate flow; (3) injecting air into the dialyzer; (4) waiting for dialyzer pressure to stabilize; (5) returning dialyzer to original position; (6) timing the refilling of the internal duct in the dialyzer by stop watch for exactly 60 seconds; (7) reestablishing dialysate flow; (8) clearing air from dialyzer by rotating it. PA1 Disadvantages: (1) injecting air; (2) special dialyzer; (3) split-second timing; (4) cessation of dialysis during the "reading"; (5) reading indicates intermittent ultrafiltration during 60-second period only--not for entire dialysis time, not even for several minutes. It is an instantaneous value which may or may not represent the average ultrafiltration. PA1 Operator care and skill are all important. PA1 Provides arterial and venous pressure, and dialysate pressure, gauges only--Transmembrance Pressure (TMP) must be calculated, including use of a permeability factor for the dialyzer. PA1 Disadvantages: (1) accuracy highly questionable; (2) complicated mathematical calculations discouraging to the user, and results not predictably precise or reproducible; (3) dialyzer variability is unknown factor. Operator care and skill are all important. PA1 The Monitor consists of a vessel to house the coil dialyzer and to plug into the canister, with a rachet mechanism to raise and lower the coil manually. During ultrafiltration measurement, and rachet is manually actuated to position the coil out of the normal dialysate perfusion circuit by valves. The ultrafiltrate then causes excess fluid to fill a sampling tube in the monitor. This is timed by stop watch and the amount accumulated translated into ml/hr. by a conversion chart. PA1 Disadvantages: (1) split-second timing; (2) cessation of dialysis during measurement; (3) reading indicates instantaneous value, which may not be representative of ultrafiltration rate over longer periods. PA1 Operator still all-important. PA1 This is a complex instrumentation system which "predicts" rate of ultrafiltration. Instruments measure the arterial pressure, the venous pressure, and the dialysate pressures to and from the dialyzer. These four values are computed by electronics to determine the Transmembrane Pressure (TMP). An arbitrary member, called the dialyzer's "Ultrafiltration Index", is then factored in, and a calculated result is called the UFR or Ultrafiltration Rate. PA1 Disadvantages: Complex instrumentation which is capable only of instantaneous values, and subject to gross errors of the dialyzer's "Index". PA1 The manufacturer lists the following "several variables that can affect fluid removal. These should always be considered in assessing ultrafiltration: (1) Accuracy of the method for measuring patient weight before and after dialysis; (2) Fluid inputs and losses during priming, dialysis, and rinse back; (3) Solid and liquid food intake during dialysis; (4) Osmolality of the dialysate and blood; (5) The electrolyte balance of the patient; (6) Blood and dialysate flow rates during dialysis; (7) Variations in dialyzer membrane characteristics (the UF Index), and (8) Clinical status affecting insensible loss".
The circuit of the current invention splits off the ultrafiltrate from effluent, through a vernier bypass, AFTER the downstream pump--on the positive pressure output of the negative pressure downstream pump.
The state-of-the-art today employs accurate stop-watch timing to determine the volume extracted in a precise interval, when dialysis has been temporarily bypassed; or mathematical calculations of transmembrane pressure (4 measurements) and a factor representing the dialyzer's estimated filtration characteristics. This calculation is performed electronically in some cases, or multiplied out by staff or patient in others.